1. Field of the Invention
This invention relates to a chemical process for recovering uranium, among other potentially useful or valuable materials, from ammonium nitrate-containing water or waste effluent. The method is particularly useful in procedures for converting impure uranium dioxide to uranium dioxide of a grade suitable for use as fuel for nuclear fission reactors.
2. Description of the Background Art
One conventional means of producing fission fuel grade uranium dioxide consists of a wet conversion procedure, comprising the steps or reactions of: (a) hydrolyzing gaseous uranium hexafluoride (UF.sub.6) with water to form water soluble uranyl fluoride (UO.sub.2 F.sub.2) and hydrogen fluoride; (b) introducing ammonium ions, such as by the addition of an excess of ammonium hydroxide, to cause the soluble uranyl fluoride to precipitate as insoluble ammonium diuranate ((NH.sub.4).sub.2 U.sub.2 O.sub.7); and, (c) upon separation of said insoluble precipitate from the water fraction, heating the ammonium diuranate to drive off entrained fluorides with ammonia and thereby convert the diuranate to uranium dioxide (UO.sub.2).
This basic uranium conversion process is disclosed in detail in the prior art, for example U.S. Pat. Nos. 3,394,997 and 3,579,311, and the disclosures and contents of said patents are incorporated herein by reference.
In the production of uranium dioxide nuclear fuel pellets, a substantial quantity of uranium dioxide fuel stock powder and some scrap or broken pellets are routinely recycled for reasons of economy and environmental protection. Recycling of the uranium dioxide materials is carried out by dissolving the solid uranium dioxide material in nitric acid, and thereby forming a solution of soluble uranium nitrate. The pH of a water-diluted solution of the soluble uranium nitrate is reduced to about 5 to about 7 with ammonium hydroxide, and then the uranium is precipitated from the solution by the addition of hydrogen peroxide to produce insoluble uranium peroxide according to the reaction: EQU UO.sub.2 (NO.sub.3).sub.2 +2H.sub.2 O+H.sub.2 O.sub.2 .fwdarw.UO.sub.4.2H.sub.2 O+2HNO.sub.3
Insoluble uranium peroxide is removed in a dewatering centrifuge or other solid-liquid separating means, and then calcined to form uranium dioxide powder for reuse in the production of fissionable fuel pellets. The supernatant or effluent liquid from the solids-liquid separation is passed through a clarifier to a nitrate holding tank where lime (Ca(OH).sub.2) is added for further purification including precipitating fluorides as insoluble calcium fluoride.
The effluent from the peroxide precipitation procedure comprises ammonium nitrate, peroxides, fluorides, and other assorted impurities as well as a residue of soluble and insoluble uranium compounds. The uranium compounds are typically present in the effluent in amounts sufficient to warrant recovery thereof on the basis of economics. The substantial value of any recovered uranium and/or avoidance of the high costs of safely disposing of voluminous quantities of material containing radioactive uranium justify recovery costs.